The present invention relates to improved surgical bone cement compositions and, more particularly, to a bone cement formulated for use in the low viscosity state, which enables the bone cement to penetrate the trabeculae of the bones when used to secure a prosthesis. This penetration allows a more secure interlock between the cement and the bone, and therefore there is less likely to be a failure at the bone/cement interface.
Various types of bone cements have been used for securing prostheses, but traditionally bone cements were applied digitally and therefore, it was necessary for the cement to have a doughy consistency during the time it was being applied (its working period). In the past few years, bone cement injectors have come into use. The problem often encountered is that the regular doughy cement is often too stiff to be easily extruded from the cartridge of a bone cement injector. Often the cement ends up hardening or setting up while still in the cartridge. And more important than this, the regular doughy cement does not effectively penetrate the trabeculae of bone to create a secure interlock. This leads to loosening and eventual failure of the implanted prosthesis.
Many existing bone cements consist of a liquid monomer component and a polymer powder component. Bone cements, such as ZIMMER.RTM. Bone Cement sold by Zimmer USA, Inc. and SURGICAL SIMPLEX.RTM. P sold by Howmedica, typically are comprised of a liquid component to powder component ratio of one to two (volume/weight) where the volume is in milliliters and the weight is measured in grams. When mixing the liquid and powder components, the liquid is added to the powder, not the powder to the liquid. The mixture is stirred until a dough-like mass is formed that does not stick or adhere to the rubber gloves of the operator. (This is considered the dough time.) The dough-like mass is then usually hand manipulated or kneaded to obtain a proper consistency for digital application of the cement to the bone. The cement is then applied by hand and then the prosthesis is inserted and positioned in the cement and maintained secure until the cement hardens. (This is called set time.) The viscosity of acrylic cements increases with time elapsed after mixing the monomer and polymer components. It starts out at a lower viscosity and progresses to a stiffer and stiffer mixture until it eventually hardens completely. This type of constantly changing viscosity is representative of a pseudoplastic material.
ASTM (American Society of Testing Materials) has a standard (ASTM F 451) for acrylic bone cements. In this standard, the doughing time and setting time for acrylic bone cements is standardly defined. These tests are performed at 23.+-.2 degrees C. (72 degrees F.). To measure dough time, a stop watch is started at the onset of combining powder with the liquid. The mixture is gently probed with a surgically gloved finger. In early stages of probing, it is visually noted that fibers are formed between the surface of the mix and the finger as it leaves the surface. The time at which it is first observed that the gloved finger separates clearly is known as the dough time.
The set time according to ASTM standards is also tested at 23.+-.2 degrees C. The temperature of the cement is recorded continuously again with time measured from the onset of mixing the powder and the liquid. Set time, according to ASTM standards, is the time at which the temperature of the mixture equals (T.sub.max +T.sub.ambient)/2. T.sub.max equals maximum temperature reached. T.sub.ambient equals the ambient temperature of 23.+-.2 degrees C. The set time is considered to be the time at which the mixture hardens.
Injector guns have been advocated for application of bone cement, especially in cavities such as the femoral canal, because long slender injector tips are useful for inserting the cement deep into the cavity, instead of trying to apply it by hand. Gun application also reduces the tendency to form laminations and voids in the cement and also reduces the inclusion of blood into the cement. The problem with existing cements is that they were formulated for digital use and were therefore formulated to use after dough time when the cement becomes stiff enough to handle. In order to use the standard cements in injector guns, the surgeon must attempt to extrude the cement before dough time (the time when the cement mixture does not stick or adhere to the rubber gloves of the operator), because by then the cement is getting relatively stiff. Also, by the time the cement reaches dough time, the cement will not as effectively penetrate the trabeculae of the bone to form the desired cement/bone interlock, which is the sole mechanism for anchoring implanted prostheses securely to living bone.
ASTM has a maximum dough time specified for acrylic bone cements of five minutes. This five-minute maximum again is measured at 23 degrees C. Temperatures in operating rooms are often cooler than 23 degrees C., and are often at approximately 20 degrees C. or 68 degrees F. Once the cement powder and liquid are mixed, the reaction proceeds at a slower rate at a cooler temperature. Therefore, typically dough time occurs about a minute or so later in the operating room (O.R. dough time) than under standard ASTM test conditions.
Zimmer's standard bone cement is targeted for an ASTM dough time of about 11/2 to 2 minutes. The O.R. dough time would then be approximately at 21/2 to 3 minutes from the onset of mixing. Actual mixing of the powder and liquid usually takes about one minute. This leaves only 11/2 to 2 minutes to finish preparing the bone cement injector and extruding the bone cement in order to use it before O.R. dough time. Other existing acrylic bone cements typically have a comparable O.R. dough time of about three minutes or less. By dough time, the cement is becoming stiff enough that it becomes difficult to extrude the cement from the cartridge and tip. Problems are often encountered when using the cement in injector guns in this doughy, more viscous state, but it is attempted. The potential for secure fixation is less than optimal under these circumstances. As stated before, the cement often ends up becoming too stiff to extrude and often ends up hardening in the cartridge. Needless to say, this is undesirable and very risky under operating room conditions.
Another type of bone cement is disclosed in U.S. Pat. No. 4,064,566 which describes a graphite fiber reinforced cement. The patent speaks of preparing a slurry of the polymer powder in the liquid monomer and then adding the graphite fibers and curing agents to the mixture. The graphite fibers strengthen and reinforce the cement. The patent indicates that the fibers are added at the time the polymer and monomer components are ultimately mixed, and does not indicate that the fibers are pre-mixed in the polymer powder.